This invention relates in general to inspection of integrated circuit wafers, and more particularly, to a method for automatically inspecting a patterned wafer.
In view of the time consuming nature of manual inspection of semiconductor wafers, a great emphasis has been placed on automating all inspection operations. Recently, inspection of unpatterned starting wafers for cleanliness has been automated. Patterned wafers, however, continue to be inspected manually. Manual inspection usually involves microscopic inspection of a handful of sights on a semiconductor wafer using ordinary white light and less often using dark field microscopes. Manual inspection operations are performed several times during each photolithography step in addition to numerous inspections which occur after processing is complete.
Defect density is known to be a major yield limit in semiconductor manufacture and must be monitored to provide data for yield control. Accurate defect density measurements can also be used to predict reliability and lifetime of an integrated circuit. Unfortunately, due to the time consuming nature of manual inspection only a few circuits out of several hundred, or thousand, which are formed on a single wafer are ever inspected. Further, as the circuits become more complicated and patterns become smaller, it becomes increasingly difficult to see defects, let alone classify such defects. Present methods of integrated circuit inspection provide only estimates of defect density and thus can not fulfill the greater needs of the semiconductor industry.
Although manual inspection is rather simple and requires relatively low cost equipment, the results are somewhat inconsistent because of the subjective nature of the assessment and the attention span of the operator. Further, the time required to process the wafers as well as the limited amount of information that may be readily obtained limits the application of manual inspection techniques to statistical sampling. If such an inspection were to be carried out on all of the processed wafers, astronomical cost inefficiencies would result. In practice, this detection procedure is carried out on only a small percentage of the processed wafers. Such a procedure is grossly inefficient in that 90% or more of the processed circuits are never inspected.
Thus, a need has developed in the semiconductor industry to provide an automatic processed semiconductor wafer inspection system which can inspect all of the circuits of a large number of wafers in a time efficient manner and can classify defects which are found in the integrated circuits.
Accordingly, it is an object of the present invention to provide an improved time efficient integrated circuit inspection method.
Another object of the present invention is to provide an integrated circuit inspection system with a high immunity to variable light conditions such as lighting intensity, magnification, and rotation of the circuits under inspection.
A further object of the present invention is to provide an integrated circuit inspection system which can classify defects and anomalies on the surface of the integrated circuit.
Still another object of the present invention is to provide a method of inspecting pattered integrated circuits using direction edge enhanced images of the integrated circuit.
A further object of the present invention is to provide a method of inspecting semiconductor wafers and classifying defects thereon using a plurality of images of the defect obtained by varying light conditions.